When a vehicle is traveling in rainy or snowy weather, the outside mirrors are clouded with water droplets or ice, degrading the rearward view and therefore lowering the safety of driving. To prevent this, various types of mirrors have been proposed; which can be heated to remove water droplets and ice adhering to the mirror surface.
For example, Japanese Utility Model Publication No. 58-28937/1983 discloses a mirror for a vehicle, in which a heat distribution plate with high heat conductivity is attached to the back of a mirror base plate and has a heating body bonded to the back of the heat.
Further, Japan Utility Model Publication No. 62-33648/1987 discloses a mirror with heater, in which a flat heater is fixed to the back of a mirror body and the pattern of the heater is made more dense in the peripheral portion of the mirror than in the center.
Further, Japanese Utility Model Publication No. 102599/1992 discloses a flat heating body for a mirror, in which a heating region is divided into sections by electrodes.
The above-mentioned mirror and flat heating body for a mirror adopts a structure in which an electric heating plate which has a complex heating resistor pattern or a complex electrode pattern is fixed to the back of the mirror base plate in order to heat the entire mirror surface evenly to provide a good view. By the method using the electric heating plate, which is provided separately from the mirror base plate, it is necessary to design and manufacture a complex heating resistor pattern and electrode pattern, which increases the cost. Another drawback of this method is that because the mirror base plate is heated through the conduction of heat from the separate electric heating plate, the heat efficiency is low and it takes a long time to remove water droplets.
To solve the above problems, Japanese Utility Model Laid-Open No. 5-13872/1993 proposes a mirror with a heater, in which chromium or NICHROME is deposited on the surface of the mirror base plate by vacuum vapor deposition or sputtering to form a reflective heating resistor film whose surface is coated with an insulating overcoat layer.
Ordinary mirror reflection films are made of such materials as aluminum and chromium deposited by vacuum vapor deposition and sputtering.
It is, however, difficult to use an aluminum or chromium film as the reflective film-cum-heating resistor (reflective heating resistor film) of the mirror with a heater. The reason for this is that the electrical resistivity of aluminum and chromium is low. That is, a film made of aluminum or chromium has a low resistance, which allows a large current to flow, increasing the power consumption and making the temperature control difficult.
One possible method of solving this problem is to raise the resistance of the film made of aluminum or chromium, that is, to reduce the thickness of the aluminum or chromium film formed as the reflective heating resistor film as much as possible.
When a mirror with a heater is used for a vehicle, the current applied to the mirror is preferably in a range of 1 to 5 A. If the current is under this range, the mirror may lack the ability to melt ice in the cold season, especially when exposed to wind; and if the applied current is over this range, the current application by temperature control function may result in overheat due to overshoot, burning of peripheral components and even a human. Considering the fact that in the case of vehicles a voltage of DC 12 V is applied to a mirror with a heater, the sheet resistance of the reflective heating resistor film of the mirror is preferably in the range of 4-20 .OMEGA./.quadrature. to enable uniform heating of the mirror irrespective of its shape.
Considering the above, it is therefore possible to use aluminum or chromium for the heating resistor of the mirror with a heater for vehicles if the film thickness is set below 0.01 .mu.m when aluminum is used for the reflective heating resistor film and If the film thickness is set below 0.03 .mu.m when chromium is used. With such a thin film, even though the film is made of metal, transmission of light through the film cannot be ignored and the mirror works as a half-mirror rather than as a reflective mirror, raising a problem that depending on how light falls on to the mirror, the back side may be seen through the film thereby, degrading the view of vision of the mirror. Further, though electrodes for applying current and heating the reflective heating resistor film are attached to the film, the adhesion of the chromium film to the electrodes is poor.
Another method of solving the above problem may be to use a material for the film which has a higher electrical resistivity than aluminum and chromium.
Materials with high electrical resistivity include silicides such as NICHROME, chrome silicide and titanium silicide.
NICHROME, however, has a poor adhesion to electrode materials and consequently it is hard to achieve a stable performance. The chromium silicide film needs to be at least about 1 .mu.m thick to conduct a desired heating current but the film itself easily cracks due to stresses and the mirror base plate such as of glass may break during heating. This phenomenon is particularly noticeable in a concave mirror in which residual bending stress remains in the glass plate. Moreover, silicides generally have a low reflectivity (reflection factor) of around 30%, and at such a low level of reflectivity the function as a reflection film of the mirror cannot be fulfilled.
Further, the heating resistor is restricted by its temperature coefficient of resistance. When the temperature coefficient of resistance is too large, the heater resistance increases with an increasing temperature and reduces the current, it takes a long time for the mirror to be heated to a desired temperature, making it impossible to completely remove water droplets and ice. When, on the contrary, the temperature coefficient of resistance is too small, the current application by temperature control function may result in overheat due to current overshoot, burning peripheral components and even humans.
When a reflective heating resistor film is formed on the surface of the mirror base plate, only the central part of the mirror is easy to heat. For uniform heating of the entire mirror surface, conventionally the electrodes are provided near the peripheral portion of the mirror base plate. This method is often not effective. Mirrors for cars generally have a mirror base plate of a figure, not a circle nor rectangle, but generally parallelogram, trapezoid, oval and diamond having a narrow angle portion whose interior angle defined by the edges of the mirror base plate is small and a wide angle portion whose interior angle is large. When such a mirror base plate is used, the wide angle portion is more likely to be heated. To quickly remove water droplets in the narrow angle portion that is difficult to heat, a large amount of electricity is required. Not only is this inefficient but it may also overheat the wide angle portion, burning and deforming peripheral components such as resin holders and even burning a human when he or she touches the mirror.
As described above, the mirror with a heater disclosed in Japanese Utility Model Laid-Open No. 13872/1993 does not meet the expectations in quality.